Process for the production of oxidic protective layers on magnesium or its alloys



Patented Aug. 14, 1945 -PROCESS FOR THE PRODUCTION OF ox-' IDIC PROTECTIVE LAYERS ON MAGNE- SIUM OR ITS ALLOYS Gerhard Elssner and Ernst Schriider, Leipzig,

Germany; vested in the Alien Property Custo- N.- Drawing. Application April 10, 1939, Serial No. 267,207. In Germany April 9, 1938 2 Claims. (Cl. 20442) (Granted under the provisions of sec. 14, act of March 2, 1927; 857 O. G. 5)

It is known to protect objects of magnesium and its alloys against corrosion by providing the surfaces of the Objects with oxidic protective layers by anodic treatment in electrolytes, and subsequently, these impregnating layers, e. g. with lacquers, oils, fats and the like. Also dyeing of these oxidlc protective layers has already been carried out. Further it is known to submit the objects to be anodically oxidised to a pretreatment which comprises the generally usual degreasing of the surface and also the removal of oxides and other impurities by etching in acids, usually nitric acid. The surface of the magnesium becomes hereby or remains shining white.

In an attempt to increase the absorptive power, and the adhesiveness of the oxidic protective layers for m regnating substances such as lacquers, oils, fats or dyestuffs without thereby affecting the hardness of the oxide layers series of ably the surface of the part to be oxidised, by

other than the hitherto usual pretreatment processes. Surprisingly it has been found that a treatment with nearly neutral salt solutions, preferably with ammonium salts, e. g. ammonium chloride, gives a completely different result from the hitherto usual etching with nitric acid of diiferent concentrations. The surface does not remain polished but becomes slightly matt and bright grey without however a loose etching residue occurring on the surface.

The pure salt solutions can,als o contain small amounts of free alkalies, or acids, however the solutions must be approximately neutral. A solution which may be used for almost all purposes can be composed of 200 g. of ammonium chloride per litre of water. It is used if necessary at zlgltly raised temperatures for example 30 to For brightening the protective layers accord-' ing to the invention, a treatment in strong alkali solution is intercalated between the pretreatment dic oxidation in order to still further improve the brightening.

, above named ammonium chloride solution, these It appears to be a question of selective solution of single alloy components in the Pretreatment with salt solutions according to the invention.

Practically the action in each case is very strong and striking. If for example magnesium parts are etched in nitric acid solution and then anodically oxidised, they assume only a weak colour in the solution of an organic dyestuil'.

If however, similar parts are treated in the parts show saturated, deep and uniform colorations. I'he coloration gives a measurement of the absorptive power. Other impregnating substances also are correspondingly taken up in a strong degree. We claim:

1. In a process of providing articles of magnesium with an oxidic layer by anodic treatment in electrolytes and subsequent treatment with an impregnating medium that improvement which consists in the step of subjecting the articles to the action of a solution composed of about 200 grams or ammonium chloride per liter of water,

in the absence of an electric current prior to the.

anodic oxidation.

2. In a process of providing articles of magnesium with an oxidic layer by anodic treatment in electrolytes and subsequent treatment with an impregnating medium that improvement which consists in the stepof subjecting the articles to the. action of a solution composed of about 200 grams of ammonium chloride per liter of water, the temperature of the solution being about 30 C.

prior to the anodic oxidation.

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